i.l THE CHOICE OF AN EXAMPLE 61 



culty in showing that in this extremely complicated ap- 

 paratus all the elements are marvelously co-ordinated. 

 In order that vision shall operate, says the author of a well- 

 known book on Final Causes, "the sclerotic membrane 

 must become transparent in one point of its surface, so 

 as to enable luminous rays to pierce it . . . ; the cornea 

 must correspond exactly with the opening of the socket 

 . . .; behind this transparent opening there must be 

 refracting media . . .; there must be a retina 1 at the 

 extremity of the dark chamber . . .; perpendicular to 

 the retina there must be an innumerable quantity of trans- 

 parent cones permitting only the light directed in the line 

 of their axes to reach the nervous membrane, " * etc. etc. In 

 reply, the advocate of final causes has been invited to 

 assume the evolutionist hypothesis. Everything is mar- 

 velous, indeed, if one consider an eye like ours, in which 

 thousands of elements are coordinated in a single function. 

 But take the function at its origin, in the Infusorian, where 

 it is reduced to the mere impressionability (almost purely 

 chemical) of a pigment-spot to light: this function, pos- 

 sibly only an accidental fact in the beginning, may have 

 brought about a slight complication of the organ, which 

 again induced an improvement of the function. It may 

 have done this either directly, through some unknown 

 mechanism, or indirectly, merely through the effect of 

 the advantages it brought to the living being and the hold 

 it thus offered to natural selection. Thus the progressive 

 formation of an eye as well contrived as ours would be 

 explained by an almost infinite number of actions and re- 

 actions between the function and the organ, without the 

 intervention of other than mechanical causes. 

 The question is hard to decide, indeed, when put di- 



» Paul Janet, Les Causes finales, Paris, 1876, p. 83. 

 » Ibid. p. 80. 



